[flang][hlfir] work towards handling char_convert in hlfir

This patch aims to address the TODO for handling character conversion in HLFIR found [here](https://github.com/llvm/llvm-project/blob/1defa781243f9d0bc66719465e4de33e9fb7a243/flang/lib/Lower/ConvertExprToHLFIR.cpp#L1388) using [this similar operation but for FIR as inspiration](https://github.com/llvm/llvm-project/blob/3ea673a97b0583affc22345b9d62e863ba36b3d8/flang/lib/Lower/ConvertExpr.cpp#L1212-L1271).

Reviewed By: vzakhari, tblah

Differential Revision: https://reviews.llvm.org/D155650